1. Field of the Invention
The present invention relates to a torque detector for detecting torque without direct contact when an external force is applied to the rotating shaft of an automobile power-steering mechanism, etc.
2. Description of the Related Art
In an automobile power-steering mechanism, it is necessary to detect the amount of torque being applied to the steering wheel to determine the amount of power assistance required. Torque detectors for this purpose have been proposed, such as that disclosed in Utility Model Laid Open No. 1-180737, for example.
As shown in FIG. 6, the torque detector comprises: an upper shaft 101, which is attached to a steering wheel (not shown); a lower shaft 102, which is attached to the pinion gear of a steering mechanism (not shown); a torsion bar 103, which is disposed on the central axis of the upper shaft 101 and the lower shaft 102 and is connected to the two shafts so as to be elastic in the direction of twisting; a case 105, which rotatably supports the upper shaft by means of a bearing 104; a bobbin 106, which is disposed within the case 105; first and second movable magnetic cylinders 107, 108, which are composed of soft ferromagnetic material and are secured to the upper shaft; third and fourth movable magnetic cylinders 109, 110, which are composed of soft ferromagnetic material and are secured to the lower shaft; and first to fourth coils 111, 112, 113, 114, which are wound around the bobbin within the case 105.
The first and third movable magnetic cylinders 107, 109 sit side by side in the axial direction and first and third toothed portions 107a, 109a are disposed in the respective facing edges. The first coil 111 is disposed so as to surround these toothed portions. The second and fourth movable magnetic cylinders 108, 110 also sit side by side in the axial direction and second and fourth toothed portions 108a, 110a are disposed in the respective facing edges. The second coil 112 is disposed so as to surround these toothed portions. The third and fourth coils 113, 114 are disposed so as to surround the third and fourth movable magnetic cylinders 109, 110, which are secured to the lower shaft 102.
Next, the operation of the above conventional example will be explained. When torque is applied to the upper shaft 101 by the steering wheel, torsional deformation occurs in the torsion bar 103, and angular shear occurs between the first movable magnetic cylinder 107 and the third movable magnetic cylinder 109 and between the second movable magnetic cylinder 108 and the fourth movable magnetic cylinder 110, which are attached to the upper shaft 101 and the lower shaft 102, respectively.
Firstly, to explain the operation between the first and third movable magnetic cylinders 107, 109, the surface area of the overlap which forms a magnetic circuit between the first and third toothed portions 107a and 109a disposed on each of the movable magnetic cylinders changes, resulting in a change in the inductance in the first coil 111. The torque can be determined by detecting the change in inductance by means of a detection circuit (not shown). However, the inductance in the first coil 111 is changed not only by torque but also by temperature, and temperature is compensated for by detecting the inductance in the third coil 113, whose inductance is not affected by the twisting of the torsion bar 103.
The operation between the second and fourth movable magnetic cylinders 108, 110 is identical, so that the surface area of the overlap between the second and fourth toothed portions 108a and 110a changes, resulting in a change in the inductance in the second coil 112. The torque is determined by detecting the change in inductance. Temperature is compensated for by detecting the inductance in the fourth coil 114.
Thus, this dual construction comprising a first detection set comprising the first and third movable magnetic cylinders 107, 109 and the first and third coils 111, 113, and a second detection set, which is capable of exactly the same measurements, comprising the second and fourth movable magnetic cylinders 108, 110 and the second and fourth coils 112, 114, performs a dual safety function which enables the system to operate on the output of one of the detection sets when the other malfunctions, such as by wire breakage, etc.
The conventional torque detector requires a torsion bar 103, which is an elastic member which deforms in proportion to the torque, in addition to the first to fourth movable magnetic cylinders 107 to 110, which change the inductance in the first and second coils 111, 112, and the large number of parts makes the construction complicated.
Also, each of the toothed portions 107a to 110a on the first to fourth movable magnetic cylinders 107 to 110 are of complicated shape and require manufacture by cutting, and manufacture is therefore troublesome and expensive.
In addition, simultaneous provision of a temperature compensation function and a dual safety function, which guards against wire breakage in the coils, etc., basically requires four coils. The conventional example required two pairs of movable magnetic cylinders provided with toothed portions corresponding to the four coils.